x86/emul: correct the IDT entry calculation in inject_swint()

The logic, as introduced in c/s 36ebf14ebe "x86/emulate: support for emulating
software event injection" is buggy.  The size of an IDT entry depends on long
mode being active, not the width of the code segment currently in use.

In particular, this means that a compatibility code segment which hits
emulation for software event injection will end up using an incorrect offset
in the IDT for DPL/Presence checking.  In practice, this only occurs on old
AMD hardware lacking NRip support; all newer AMD hardware, and all Intel
hardware bypass this path in the emulator.

While here, fix a minor issue with reading the IDT entry.  The return value
from ops->read() wasn't checked, but in reality the only failure case is if a
pagefault occurs.  This is not a realistic problem as the kernel will almost
certainly crash with a double fault if this setup actually occured.

This is CVE-2016-9377 / part of XSA-196.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 255e8fe95f22ded5186fd75244ffcfb9d5dbc855
master date: 2016-11-22 13:50:49 +0100

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index 8e7f8ab41731292329907eafb37af1ff1fd5aae3..5b4fe79fa528fc9ab25d31538058c98891707cff 100644 (file)
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1451,10 +1451,16 @@ static int inject_swint(enum x86_swint_type type,
     {
         if ( !in_realmode(ctxt, ops) )
         {
-            unsigned int idte_size = (ctxt->addr_size == 64) ? 16 : 8;
-            unsigned int idte_offset = vector * idte_size;
+            unsigned int idte_size, idte_offset;
             struct segment_register idtr;
             uint32_t idte_ctl;
+            int lm = in_longmode(ctxt, ops);
+
+            if ( lm < 0 )
+                return X86EMUL_UNHANDLEABLE;
+
+            idte_size = lm ? 16 : 8;
+            idte_offset = vector * idte_size;
 
             /* icebp sets the External Event bit despite being an instruction. */
             error_code = (vector << 3) | ECODE_IDT |
@@ -1482,8 +1488,9 @@ static int inject_swint(enum x86_swint_type type,
              * Should strictly speaking read all 8/16 bytes of an entry,
              * but we currently only care about the dpl and present bits.
              */
-            ops->read(x86_seg_none, idtr.base + idte_offset + 4,
-                      &idte_ctl, sizeof(idte_ctl), ctxt);
+            if ( (rc = ops->read(x86_seg_none, idtr.base + idte_offset + 4,
+                                 &idte_ctl, sizeof(idte_ctl), ctxt)) )
+                goto done;
 
             /* Is this entry present? */
             if ( !(idte_ctl & (1u << 15)) )